At present, most teleconferencing systems use a single full-duplex audio channel for voice communications. These systems also make use of an acoustic echo canceller to reduce the undesired echo resulting from the coupling between the loudspeaker and the microphone. To make these systems more lifelike, better and more realistic sound systems are required. High fidelity wide bandwidth (100 to 7000 Hz) voice communication system is now being used. However, in order to introduce spatial realism, more than one channel is needed. Therefore, future teleconferencing systems are expected to have more than one channel (at least stereo with two channels) of full duplex voice communications.
One of the fundamental problems in stereophonic acoustic echo cancellation (AEC) systems is that given the input to the loudspeakers and the output of the microphones in the receiving room, the echo path cannot be determined uniquely. See for example the following references: J. Benesty, D. R. Morgan and M. M. Sondhi, “A Better Understanding and an Improved Solution to the Problems of Stereophonic Acoustic Echo Cancellation,” Preprint, Proceedings of ICASSP-97, Vol. 1, pp. 303-306, Munich, Germany, Apr. 21-24, 1997; J. Benesty, P. Duhamel and Y. Grenier, “Multi-Channel Adaptive Filtering Applied to Multi-Channel Acoustic Echo Cancellation,” Preprint, Submitted to IEEE Trans. on Signal Processing, April 1995; S. Shimauchi and S. Makino, “Stereo Projection Echo Canceller with True Echo Path Estimation,” Proceedings of ICASSP-95, pp. 3059-3062, 1995; and M. M. Sondhi, D. R. Morgan and J. L. Hall, “Stereophonic Acoustic Echo Cancellation—An Overview of the Fundamental Problem,” IEEE Signal Processing Letters, Vol. 2, No. 8, pp. 148-151, Auguest 1995. The problem is due to the correlation between the stereo signals. As a result, any adaptive technique used in stereophonic AEC systems fails to identify the echo path responses correctly. To circumvent this problem, it is necessary to develop techniques to decorrelate the stereo signals at the input to the loudspeakers without affecting stereo perception.
Several techniques have been proposed in the past, e.g., addition of random noise, modulation of signal, decorrelation, filters, inter-channel frequency shifting etc. However, these techniques either do not correlate the signals or destroy stereo perception completely. The interleaving comb filtering proposed in Sondhi et al. cited above only gives partial identification (above 1 kHz) of the echo path responses. Recently, a technique is proposed in Benesty et al. cited above based on non-linear processing of the stereo signals. However, as noted by the authors of Benesty et al., for tonal signal, the technique based on non-linearly cannot maintain transparency in perception (changes the pitch perception).